The diversity of the cytochromes P-450 in human tissues, i.e., placenta, monocytes and lymphocytes was investigated using monoclonal antibodies (MAbs) to 3-methylcholanthrene (MC) and phenobarbital (PB)-induced rat liver cytochrome P-450. MAbs to rat liver MC-induced cytochrome P-450 not only inhibited the aryl hydrocarbon hydroxylase (AHH) of MC-induced rat liver microsomes but also inhibited AHH of human placenta and lymphocytes. The 7-ethoxycoumarin deethylase activity of human placenta and lymphocytes was also inhibited. The degree of enzyme inhibition by monoclonal antibodies was different for different individuals and tissues. The content of antigenically unique types of cytochrome P-450 responsible for different drug and carcinogen reactions can therefore be measured in different individuals by the extent of their inhibition by monoclonal antibodies.